The present invention generally relates to optical information storage apparatuses, and more particularly to an optical information storage apparatus which is suited for reproducing high-quality magneto-optic signals from a recording medium.
In this specification, an "information storage apparatus" refers to an apparatus which records information to and/or reproduces information from a recording medium.
In an optical information storage apparatus, a light beam emitted from a light source is reflected by a plate-shaped polarization beam splitter and irradiated on a recording surface of an optical recording medium. The light beam which is reflected by the recording surface of the optical recording medium passes through the plate-shaped polarization beam splitter and is directed towards a photodetector. An astigmatism is generated when the reflected light beam from the optical recording medium passes through the plated shaped polarization beam splitter. In a compact disk player which reproduces a signal from a compact disk (CD), for example, this astigmatism is used to generate a focal error signal.
FIG. 1 is a plan view showing the general construction of an example of an optical system of the CD player. In FIG. 1, a light beam emitted from a semiconductor laser 14 is input to a diffraction grating 15, and .+-.1st order lights which are sub light 15 beams used to detect a tracking error are generated. The light beam passing through the diffraction grating is subjected to an amplitude division by a semitransparent mirror 16, and is reflected by the semitransparent mirror 16 depending on a reflectivity of the semitransparent mirror 16 and then input to a collimator lens 17. The collimator lens 17 converts the incoming light beam into parallel rays. The parallel rays from the collimator lens 17 are reflected in a direction perpendicular to the paper in FIG. 1 by a mirror 18 and reach an objective lens 19 which stops the rays to a diffraction limit, thereby irradiating a pit 21 provided on the CD. In FIG. 1, an arrow 20 indicates a direction of an electric vector of the light irradiated on the CD. In the case of the CD, the direction 20 of the electric vector of the light irradiated on the CD is not very important, but this direction 20 is set to a 45.degree. direction with respect to a row of the pits 21.
The light reflected by the pit 21 is again input to the objective lens 19, and is passed through the collimator lens 17 to become a convergent light. This convergent light is subjected to an amplitude division by the semitransparent mirror 16, and is transmitted through the semitransparent mirror 16 depending on a transmittance of the semitransparent mirror 16. The light transmitted through the semitransparent mirror 16 generates an astigmatism and a coma aberration. The coma aberration is eliminated and only the astigmatism is extracted from the light transmitted through the semitransparent mirror 16, by passing the light from the semitransparent mirror 16 through a plano-concave lens 22 which is inclined in a direction opposite to the inclination of the semitransparent mirror 16. The light which passes through the plano-concave lens 22 is detected by a photodetector 23a, and a radio frequency (RF) signal, a focal error signal and a tracking error signal are generated based on an output of the photodetector 23a.
FIG. 2 is a plan view showing the general construction of the photodetector 23a on an enlarged scale. The photodetector 23a includes a 4-part detector 24a which is made up of detector parts A, B, C and D, a detector 25a, and a detector 26a. The RF signal is generated from a sum A+B+C+D of output photocurrents A, B, C and D of the detector parts A, B, C and D of the 4-part detector 24a. The focal error signal is generated from a difference between a sum A+C of the output photocurrents A and C of the detector parts A and C of the 4-part detector 24a, and a sum B+D of the output photocurrents B and D of the detector parts B and D of the 4-part detector 24a.
The tracking error signal is generated from a difference between an output photocurrent of the detector 25a and an output photocurrent of the detector 26a. A push-pull signal which appears in a radial direction of the CD depending on the row of the pits 21 provided on the CD is generated in a direction indicated by an arrow in FIG. 2. This push-pull signal is not used as a tracking error signal.
As shown in FIG. 1, the CD player is designed so that the direction in which the astigmatism is generated is inclined by approximately 45.degree. with respect to a direction in which the signal of the CD flows. This design is employed because the push-pull signal appears in the radial direction of the CD due to the diffraction phenomenon when a spot of the light beam which is stopped to the diffraction limit is irradiated on the pit 21 of the CD. A frequency of this push-pull signal is determined by an amount of decentering, a rotational speed and a track pitch of the CD. A frequency band of the push-pull signal is approximately the same as a frequency band of the focal error signal which is obtained by the astigmatism method.
On the other hand, FIG. 3 is a diagram showing a case where a direction in which the astigmatism is generated in the CD player is parallel to a direction in which the signal of the CD flows. In addition, FIG. 4 is a plan view showing the general construction of a photodetector 23b. In FIG. 3, those parts which are the same as those corresponding parts in FIG. 1 are designated by the same reference numerals, and a description thereof will be omitted.
In this case, the tracking error signal is generated from a difference between output photocurrents of detectors 25b and 26b of the photodetector 23b, and no inconvenience is introduced. However, detector parts A through D of a 4-part detector 24b that are used to generate the focal error signal must be arranged as shown in FIG. 4. Accordingly, the direction in which the astigmatism is generated becomes parallel or perpendicular to the direction in which the push-pull signal is generated, and division lines (dark lines) of the 4-part detector 24b are inclined by 45.degree. with respect to the direction in which the push-pull signal is generated. For this reason, the push-pull signal easily mixes into the focal error signal as a crosstalk, and a stable focal error signal cannot be obtained.
In the case of a recording medium, such as the CD, exclusively for use when reproducing signals from the recording medium, no inconvenience is introduced regardless of the polarization direction of the light beam irradiated on the recording medium.
In the case of a magneto-optic recording medium, the polarization direction of the light beam irradiated on the recording surface of the magneto-optic recording medium at the time of the signal reproduction is extremely important. Generally, it is known that the noise is reduced when the polarization direction of the light beam is parallel or perpendicular to a tracking guide groove which is provided on the magneto-optic recording medium. This noise reduction is due to the birefringence of a substrate and the shape of the groove of the magneto-optic recording medium. Accordingly, in the case of the optical information storage apparatus which reproduces the signal from the magneto-optic recording medium, there was a problem in that the push-pull signal will mix into a focal error signal as a crosstalk if an attempt is made to generate the focal error signal by the astigmatism method using a plate-shaped polarization beam splitter as in the case of the CD player shown in FIG. 3, and that it is extremely difficult to generate a stable tracking error signal. On the other hand, in the case of the magneto-optic recording medium, there was a dilemma in that it is extremely important to make the polarization direction of the light beam irradiated on the recording surface parallel or perpendicular with respect to the groove of the magneto-optic recording medium.